The Effects of Speech Intelligibility an

The Effects of Speech Intelligibility and Background Noise on the Comprehension of Accented Speech

Natacha Moreno, M.S., CCC-SLP

When it comes to processing speech, research has clearly documented the adverse effect of noise on the listening environment. Speech simply becomes more difficult to hear, and as a result, listeners must rely on additional cognitive resources to cope with the acoustic challenge. As a result, this effortful listening can interfere with attention, language, and memory processes – all key ingredients for effective communication.

Noise, however, is not the only factor that can negatively impact the listener’s ability to process speech. In 2014, researchers Han-Gyol Yi, Rajka Smiljanic and Bharath Chandrasekaran, studied the cognitive effects of accented speech on the listener and found that foreign-accented speech perception can also place greater demand on the neural systems underlying speech perception.

But what happens to our ability to process speech when there is interplay between different degrees of foreign- accented speech and varying levels of background noise? In 2010, researchers Erin Wilson and Tammie Spaulding set out to examine the effects of both noise and speech intelligibility on listener comprehension and processing time of Korean-accented English. In this study, intelligibility refers to the accuracy in which a listener deciphers a speaker’s message, while comprehension is the listener’s ability to interpret and understand the speaker’s intended message.

54 monolingual, English–speaking adults served as listeners for this study. Speakers included two native English individuals, two Korean-accented speakers determined to be highly intelligible, and two Korean-accented speakers with moderate intelligibility (as rated from a sentence transcription task.). 36 sentences were produced by the three speaker groups, half true and half false, and were presented to the listeners in each noise condition - quiet, +10 dB signal-to-noise ratio (speech signal +10 db above the noise) and +5 dB SNR. Comprehension was determined by true/false accuracy of spoken statements, and the processing time was estimated by response latency, in this case, the elapsed time of when the listener hears the statement and when he/she responds to it.

Results were as follows:

The ability to comprehend what was spoken decreased with decreasing speech intelligibility (greater accentedness) and increasing levels of background noise.

There was a 34% decrease in listener comprehension of Korean accented speech in +10 db SNR (signal to noise ratio) when compared to quiet conditions, indicating that even a modest amount of noise differentially affected listeners’ comprehension of accented speech. In contrast, there was only a 3% (non-significant) decrease in comprehension for native English speech in this same listening condition

In the most challenging listening condition of a +5 dB signal to noise ratio, the listeners’ comprehension for speech was significantly decreased across all speaker groups.

The time required to process speech increases as a function of decreasing speech intelligibility. Korean-accented speech of moderate intelligibility incurred the longest response latencies when compared to native English and highly intelligible Korean-accented speech.

The outcomes of this study have important implications for optimizing verbal communication between foreign-accented speakers and native listeners in different work settings. For example, verbal exchanges may be most effective in the least noisy spaces. Additionally, foreign-accented speakers may choose to slow down their rate of speech to allow for more processing time on the part of the listener. Lastly, amplification may be a favorable option for foreign-accented individuals when presenting to a group.

Wilson, E. O., & Spaulding, T. J. (2010). Effects of noise and speech intelligibility on listener comprehension and processing time of Korean-accented English. Journal of Speech, Language, and Hearing Research, 53, 1543–1554.

Yi, Han-Gyol, and Rajka Smiljanic. "The Neural Processing of Foreign-accented Speech and Its Relationship to Listener Bias." Frontiers. Frontiers in Human Neuroscience, 08 Oct. 2014. Web. 19 Dec. 2014.